I do see a lot of jokes and some fun coming from this game-mode, but I don't thing it'll be a thing, at least in the near future for a few reasons.
The first is because, while it does sound funny, it doesn't fit any sort of thematic aside from "What if everyone was a yordle". Most game-modes, or at least events, have a theme to them, whether it be blood moon, to star guardian, to URF. This one would be just out of the blue.
The second is because of the amount of time this would take. If this were to become a thing, RIOT would have to make completely new models for each character to make their proportions and everything work. The work that would have to go into this game-more, to put it into a simple context, would be possibly equal to making a chibi skin for nearly every champ, (Excluding already yordle champs), along with possibly even new animations and modified particle effects to accommodate the change in size. Not to mention that, if they allowed skins in this game-mode, they would have do do the same thing for every single skin enabled in the new mode.
Along with the second point, it would also increase the game size dramatically because all of these new models and animations would be added on to the already existing files.
Again, it is pretty funny, but I feel like if this were to be made, the joke would die in about 2-3 weeks, maybe 5, then it'd just be there sorta like how dominion was. Some people would continuously play it, but it may not have the same kind of traction that the other maps do.